These benchmarks are informal and only intended to give us a general sense of the benefit Dart Sass could provide relative to other implementations.

This was tested against:

libsass 27437bc and sassc 36eb82e compiled with g++ 4.8.4.
Dart Sass 2bda8fa on Dart 1.22.0-dev.10.3 and Node 7.2.0.
Ruby Sass e79f5cf on Ruby 2.2.4p230.

on Ubuntu x64 with Intel Xeon E5-1650 v3 @ 3.50GHz. The Dart Sass application snapshot was trained on the preceding_sparse_extend.scss file.

Measurements

I ran five instances of each configuration and recorded the fastest time.

Small Plain CSS

Running on a file containing 4 instances of .foo {a: b}:

sassc: 0.006s
Dart Sass from source: 0.278s
Dart Sass from a script snapshot: 0.206s
Dart Sass from an app snapshot: 0.072s
Dart Sass on Node.js via dart2js: 0.246s
Ruby Sass with a hot cache: 0.130s

Large Plain CSS

Running on a file containing 2^17 instances of .foo {a: b}:

sassc: 2.178s
Dart Sass from source: 2.341s
Dart Sass from a script snapshot: 2.291s
Dart Sass from an app snapshot: 2.099s
Dart Sass on Node.js via dart2js: 5.758s
Ruby Sass with a hot cache: 14.484s

Based on these numbers, Dart Sass from an app snapshot is approximately:

identical to libsass
2.7x faster than Dart Sass on Node
6.9x faster than Ruby Sass

Preceding Sparse `@extend`

Running on a file containing .x {@extend .y}, 2^17 instances of .foo {a: b}, and then .y {a: b}:

sassc: 2.338s
Dart Sass from a script snapshot: 2.326s
Dart Sass from an app snapshot: 2.123s
Dart Sass on Node.js via dart2js: 6.082s
Ruby Sass with a hot cache: 22.423s

Based on these numbers, Dart Sass from an app snapshot is approximately:

1.1x faster than libsass
2.9x faster than Dart Sass on Node
10.6x faster than Ruby Sass

Following Sparse `@extend`

Running on a file containing .y {a: b}, 2^17 instances of .foo {a: b}, and then .x {@extend .y}:

sassc: 2.363s
Dart Sass from a script snapshot: 2.308s
Dart Sass from an app snapshot: 2.143s
Dart Sass on Node.js via dart2js: 6.045s
Ruby Sass with a hot cache: 22.221s

Based on these numbers, Dart Sass from an app snapshot is approximately:

1.1x faster than libsass
2.8x faster on the Dart VM than on Node
10.4x faster than Ruby Sass

Preceding Dense `@extend`

Running on a file containing .bar {@extend .foo} followed by 2^17 instances of .foo {a: b}:

sassc: 6.826s
Dart Sass from a script snapshot: 3.324s
Dart Sass from an app snapshot: 3.086s
Dart Sass on Node.js via dart2js: 12.054s
Ruby Sass with a hot cache: 40.193s

Based on these numbers, Dart Sass from an app snapshot is approximately:

2.2x faster than libsass
3.9x faster on the Dart VM than on Node
13.0x faster than Ruby Sass

Following Dense `@extend`

Running on a file containing 2^17 instances of .foo {a: b} followed by .bar {@extend .foo}:

sassc: 6.796s
Dart Sass from a script snapshot: 3.751s
Dart Sass from an app snapshot: 3.339s
Dart Sass on Node.js via dart2js: 11.551s
Ruby Sass with a hot cache: 39.603s

Based on these numbers, Dart Sass is approximately:

2.0x faster than libsass
3.5x faster on the Dart VM than on Node
11.9x faster than Ruby Sass

Conclusions

Based on this (admittedly imperfect and non-representative) data, Dart Sass can match the best performance of any Sass implementation. Because it eagerly tracks data for @extends, its worst case is when no @extends are present and that tracking proves unnecessary. However, even there it matches the speed of the pure-C++ LibSass implementation.

Because of the novel structuring of @extend, we see its relative performance increase along with the amount of extension. With only one @extend it's slightly faster than LibSass; with hundreds of thousands, it's vastly faster.

It's worth noting that Dart Sass implements @extend semantics according to issue 1599, while other implementations do not. This certainly simplifies the implementation and may explain some of the speed gains. However, even if other implementations could be faster, it's still the case that Dart Sass is fast enough.

The only place where Dart Sass falls behind LibSass is when processing small files—it's difficult for any VM to beat the startup speed of C++. But the app snapshot model means that it stays beneath the crucial 100ms limit for trivial files, which means it will look effectively instantaneous to humans.

It's also interesting to note where Dart Sass falls when run on Node.js. It's enough slower than the Dart VM that we probably don't want to position Node.js as the primary way of running Sass, but it's still substantially faster than Ruby. It probably makes sense to distribute Dart Sass through JS channels as an low-overhead introduction, and then make it easy for users to upgrade to the Dart version later on for more speed.

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